I use a thick steel ruler
as a spacer which results in about 1/32 inch extra. The
12 inch ruler is flexible and bent down so it butts up
tight against the rabbit. Both left and right side tenon
shoulders are marked on the rails. They are knifed
later.

Marking for rail shoulders.

On the face side, the tenon
shoulder is a quarter inch farther out to allow for
coping the molded edge. Here the back side line has been
extended up the rails side and I used a 1/4″ brass
spacer to locate the face side shoulder.

Quarter inch spacer defining face
shoulder.

This shows the offset
shoulder laid out. The face and rear shoulder lines will
be knifed to help with accurate sawing, the short side
lines are not knifed.

Rail tenon offset to allow coping.

My practice raised panels
varied a bit in depth so here I am checking the distance
between panel top surface and the bottom of the rear
rabbit. Ideally the distance between the top surface of
the panel and the bottom of the rear rabbit groove will
be 9/16″ which will allow 1/4″ panel raise, 1/4″ panel
edge thickness, and 1/16″ for the Ovolo molding.